Crowding and eccentricity determine reading rate

Abstract

Bouma's law of crowding predicts an uncrowded central window through which we can read and a crowded periphery through which we cannot. The old discovery that readers make several fixations per second, rather than a continuous sweep across the text, suggests that reading is limited by the number of letters that can be acquired in one fixation, without moving one's eyes. That "visual span" has been measured in various ways, but remains unexplained. Here we show (1) that the visual span is simply the number of characters that are not crowded and (2) that, at each vertical eccentricity, reading rate is proportional to the uncrowded span. We measure rapid serial visual presentation (RSVP) reading rate for text, in both original and scrambled word order, as a function of size and spacing at central and peripheral locations. As text size increases, reading rate rises abruptly from zero to maximum rate. This classic reading rate curve consists of a cliff and a plateau, characterized by two parameters, critical print size and maximum reading rate. Joining two ideas from the literature explains the whole curve. These ideas are Bouma's law of crowding and Legge's conjecture that reading rate is proportional to visual span. We show that Legge's visual span is the uncrowded span predicted by Bouma's law. This result joins Bouma and Legge to explain reading rate's dependence on letter size and spacing. Well-corrected fluent observers reading ordinary text with adequate light are limited by letter spacing (crowding), not size (acuity). More generally, it seems that this account holds true, independent of size, contrast, and luminance, provided only that text contrast is at least four times the threshold contrast for an isolated letter. For any given spacing, there is a central uncrowded span through which we read. This uncrowded span model explains the shape of the reading rate curve. We test the model in several ways. We use a "silent substitution" technique to measure the uncrowded span during reading. These substitutions spoil letter identification but are undetectable when the letters are crowded. Critical spacing is the smallest distance between letters that avoids crowding. We find that the critical spacing for letter identification predicts both the critical spacing and the span for reading. Thus, crowding predicts the parameters that characterize both the cliff and the plateau of the reading rate curve. Previous studies have found worrisome differences across observers and laboratories in the measured peripheral reading rates for ordinary text, which may reflect differences in print exposure, but we find that reading rate is much more consistent when word order is scrambled. In all conditions tested--all sizes and spacings, central and peripheral, ordered and scrambled--reading is limited by crowding. For each observer, at each vertical eccentricity, reading rate is proportional to the uncrowded span.

“Pseudoextinction”: Asymmetries in simultaneous attentional selection.

“…Studies of crowding (Bouma, 1970;Pelli et al, 2007;Toet & Levi, 1992) have indicated that the letters in our experiments-presented at a retinal eccentricity of 6.0°-should be free from crowding when separated by about 3.0° or more. In fact, they were separated by 8.5° (see Figure 1a), suggesting that crowding played no appreciable role in either condition.…”

Frequency Drives Lexical Access in Reading but not in Speaking: The Frequency-Lag Hypothesis

“…Assuming college-students read anywhere from 2-8 hours/day, at a rate of about 300 words/minute (Pelli, Tillman, Freeman, Su, Berger, & Majaj, 2007;Taylor, 1965), this would translate into many more words read than spoken/day 4 (300*60*2= 36,000 and 300*60*8=144,000).…”

Extra-large letter spacing improves reading in dyslexia

“…20 and 21). Recognition is impaired when letters are closer than a critical spacing (19), which is proportional to eccentricity but independent of print size (22).…”

“…Crowding mostly affects peripheral vision in normal adult readers (22), but it also affects central vision in school-aged children (23). Moreover, there is mounting evidence that children with developmental dyslexia are more influenced by crowding than agematched controls, even under optimal viewing condition (14-17, 24, 25).…”

“…Parsing of a letter string into its constituent graphemes is a key component of phonological decoding (28), which in turn, is fundamental for reading acquisition (29). Crowding might not only slow down reading speed (19,22) but also induce reading errors, because crowding is accompanied by a jumbled percept that is thought to reflect pooling of features from the target and the flankers (21). These findings lead to the prediction that extra-wide interletter spacing in words should reduce crowding and ameliorate reading performance in dyslexics.…”